The present invention relates to a catalyst for reducing nitrogen oxides and to a method of reductively eliminating dilute nitrogen oxides in exhaust gas wherein the catalyst is used together with a reducing agent. More particularly, the present invention relates to a catalyst for reducing nitrogen oxides which is derived from a naturally produced .alpha.-Fe.sub.2 O.sub.3 -containing mineral alone or in mixture with one or more of specific kinds of metals or compounds thereof as well as a method of reductively eliminating dilute nitrogen oxides in exhaust gas wherein the exhaust gas is brought together with ammonia as reducing agent into contact with the catalyst.
Exhaust gas burnt and discharged from various fixed combustion units contains harmful ingredients such as sulfur oxides, nitrogen oxides and the like. The nitrogen oxides are regarded as substances causing environmental pollution, especially photochemical smog as a new type air pollution. In view of preventing air pollution which now becomes a big social problem, therefore, it is highly desired to inhibit discharge of nitrogen oxides from combustion units.
At present, various processes for inhibiting discharge of nitrogen oxides or various denitration processes are proposed as techniques for reducing the amount of nitrogen oxides discharged. Among these processes, a selectively catalytic reduction process wherein nitrogen oxides in exhaust gas are reacted with ammonia in the presence of a catalyst or composite catalyst composed of an oxide of iron, vanadium, chromium, copper or cerium or a catalyst comprising such metal oxide supported on a carrier such as .gamma.-alumina or titanium oxide, thereby reductively converting the nitrogen oxides into harmless water and nitrogen is considered to be most effective.
As the temperature of exhaust gas from a fixed combustion unit such as a boiler is usually kept at 100-150.degree. C., however, a denitrating equipment must be operated under such condition that various factors such as the amounts and concentrations of reactants which have significant influence on the chemical reaction rate cannot be kept constant. Thus, the catalyst utilizable for this purpose requires such performances as high rate of eliminating nitrogen oxides within a wide range of temperatures especially at lower temperatures and catalytic activity hardly reducible in the coexistence of sulfur dioxide, water carbon dioxide, oxygen or dust contained in exhaust gas. In addition, the catalyst must be nontoxic and less expensive and free of such defect that the active ingredient is easily detached from the carrier. On practical use of the catalyst, most of the above requirements should be satisfied. Actually, however, a conventional catalyst for reducing nitrogen oxides in which an oxide catalyst such as iron oxide has to be supported on a carrier such as .gamma.-alumina or titanium oxide cannot economically be prepared. When such catalyst is used for a reducing treatment of nitrogen oxides in exhaust gas for a long period of time under the above mentioned high temperature conditions, .gamma.-alumina, titanium oxide and the like oxide are easily attacked or poisoned by sulfur oxides and water also contained in the exhaust gas, thus resulting in shortening of the bed life of the catalyst. Thus, there is a great demand for developing a new type catalyst for this purpose, which possesses stable high catalytic activity for a prolonged period of time and satisfactory mechanical strength.